1. Field of the Invention
The present invention relates generally to a photochromic laminate that can be applied to polymeric surfaces or can be used by itself as a photochromic element. The invention also relates to a photochromic laminate that is capable of withstanding high temperatures and can be incorporated into plastic lenses by means of injection molding. The invention further relates to a photochromic laminate that is excellent in both control of thickness and surface smoothness of the photochromic layer, and thereof exhibits uniform darkness at the activated state.
2. Description of the Related Art
Photochromic articles, particularly photochromic plastic materials for optical applications, have been the subject of considerable attention. In particular, photochromic ophthalmic plastic lenses have been investigated because of the weight advantage and impact resistance they offer over glass lenses. Moreover, photochromic transparencies, e.g. window sheets, for vehicles such as cars, boats and airplanes, have been of interest because of the potential safety features that such transparencies offer.
The use of polycarbonate lenses, particularly in the United States, is widespread. The demand for sunglasses that are impact resistant has increased as a result of extensive outdoor activity. Materials such as polycarbonate have not historically been considered optimal hosts for photochromic dyes due to slow activation rate, slow fading (bleeching) rate, and low activation intensity.
Nonetheless, there are several existing methods to incorporate photochromic properties into lenses made from materials such as polycarbonate. One method involves applying to the surface of a lens a coating containing dissolved photochromic compounds. For example, Japanese Patent Application 3-269507 discloses applying a thermoset polyurethane coating containing photochromic compounds on the surface of a lens. U.S. Pat. No. 6,150,430 also discloses a photochromic polyurethane coating for lenses.
Another method involves coating a lens with a base coating. An imbibing process described in U.K. Pat. No. 2,174,711 or U.S. Pat. No. 4,968,454 is used to imbibe a solution containing photochromic compounds into the base coating material. The most commonly used base material is polyurethane.
However, the two methods described above, which involve coating the lens after it is molded, have significant shortcomings. For example, typically a coating of about 25 μm or more is needed to incorporate a sufficient quantity of photochromic compounds into the base in order to provide the desired light blocking quality when the compounds are activated. This relatively thick coating is not suited for application on the surface of a segmented, multi-focal lens because an unacceptable segment line and coating thickness nonuniformity around the segment line are produced, and the desirable smooth surface quality is affected.
Lenses made from plastic materials such as polycarbonate are produced by an injection molding process and insert (also known as in-mold decoration) injection molding is used to incorporate photochromic properties into the lenses. Insert injection molding is a process whereby a composition is injection molded onto an insert in the mold cavity. For example, as disclosed in commonly assigned U.S. Pat. No. 6,328,446, a photochromic laminate is first placed inside a mold cavity. Polycarbonate lens material is next injected into the cavity and fused to the back of the photochromic laminate, producing a photochromic polycarbonate lens. Because the photochromic function is provided by a thin photochromic layer in the laminate, it is practical to make photochromic polycarbonate lenses with any kind of surface curvature by the insert injection molding method.
Transparent resin laminates with photochromic properties have been disclosed in many patents and publications, for example, Japanese Patent Applications 61-276882, 63-178193, 4-358145, and 9-001716; U.S. Pat. No. 4,889,413; U.S. Patent Publication No. 2002-0197484; and WO 02/093235. The most commonly used structure is a photochromic polyurethane host layer bonded between two transparent resin sheets. Although the use of polyurethane as a photochromic host material is well known, photochromic polyurethane laminates designed especially for making photochromic polycarbonate lenses through the insert injection molding method are unique.
Problems associated with conventional insert injection molding techniques in the manufacture of photochromic lens are polyurethane bleeding and poor replication of segment lines. “Bleeding” occurs from the deformation of the polyurethane layer during processing. In particular, bleeding occurs when the polyurethane layer melts and escapes from its position between the two transparent sheets of the laminate during the injection molding process. The inventors have discovered that bleeding most frequently results from an excess amount of polyurethane and from using too soft a material. The inventors have also discovered that poor replication of segment lines occurs when the layer of polyurethane is too thick and movement of the laminate occurs as pressure from the mold is applied.
In order to prevent the bleeding problem, it is preferred to have the polyurethane cross-linked. However, cross-linked polyurethane, once made, is difficult to be laminated between transparent resin sheets. A convenient method to incorporate cross-linked polyurethane is to start with a liquid polyurethane system such as the one described in U.S. Patent Publication No. 2002-0197484. To make the laminate efficiently, a web coat-laminate line such as the one described in Japan Patent Laid Open 2002-196103, is usually used. The state of the art coating equipment is capable of coating a uniform layer of liquid polyurethane mixture. However, this layer will only be partially solidified (or cured) at the moment of in-line lamination. Any possible surface defects of resin sheet and lamination rollers are easily transferred to the soft polyurethane layer during lamination. The most often seen defects in the polyurethane layer include thickness un-evenness across the web and thin spots due to uneven pressure at lamination or improper handling. In order to have the polyurethane layer firm enough to withstand the necessary pressure during lamination, it needs to be cured for a certain amount of time, which slows down the processing or renders the continues web coating-laminating impossible.
Therefore, the need exists to overcome the problems and shortcomings associated with existing polyurethane laminates having photochromic properties and methods of making these laminates.